Work the right muscle groups. Nothing will help you as much in staying low, while playing football, as weight training your quadriceps, calves, and hamstrings. Make sure that you lift weights, including a full lower body workout, at least three times a week.
Use the chutes. Football drills that are held in the chutes are hellish while you're doing them, but will help your ability to stay low more than any other drill. Practice in the chutes as often as possible, especially if you're having trouble staying low during football games.
Consult your offensive line coach. No one on your team's coaching staff will be able to help you any more than your offensive line coach because because he works with offensive linemen every day who tend to have the most trouble staying low but need to do it more than any other position. Ask your offensive line coach if he'd be willing to give you a little extra help after practice and cash in on his expertise.
Stretch every day. In order to stay low during football games you need strength, balance, and flexibility. By doing a full lower body stretching routine each and every day you'll help yourself immensely.